This disclosure relates to removing a recast layer from a metal substrate, such as a turbine blade.
Structures used in the aerospace industry, such as turbine blades, typically include complex passages that are provided in a substrate formed of a superalloy. Passages, such as cooling holes extending from an inner passage to an outer surface of the turbine blade, cannot be machined using conventional metal removal processes such as drilling. These cooling holes and other cooling passages are often formed using an electrical discharge machining (EDM) process, which results in a recast layer in the machined area. The recast layer has different metallurgical characteristics than that of the substrate. For example, the recast layer may include small cracks due to shrinkage as the molten metal from the EDM process resolidifies.
This recast layer is typically removed from the substrate. A process has been developed by which the recast layer is removed from the substrate using a chemical milling process. The chemical milling process includes submersing the substrate in an etchant, which comprises a solution of HNO3, HCl, H2O, FeCl3 and CuSo4. As the recast is removed from the substrate, gases are evolved on the surface of the substrate. Once the gases cease evolving, it was believed that all of the recast material had been removed from the substrate.